Electrophotographic technology, also referred to as xerography, involves the use of electrophotographic techniques to form images on a receptor, such as paper, film, or the like. Electrophotographic technology is incorporated into a wide range of equipment including photocopiers, laser printers, facsimile machines, and the like.
A representative electrophotographic process involves a series of steps to produce an image on a receptor, including charging, exposure, development, transfer, fusing, and cleaning, and erasure. In the charging step, a photoreceptor is covered with charge of a desired polarity, either negative or positive typically. In the exposure step, an optical system forms a latent image of charge on the photoreceptor corresponding to the image to be formed on the receptor. In the development step, toner particles of the appropriate polarity are generally brought into contact with the latent image. The toner particles adhere to the latent image via electrostatic forces. In the transfer step, the toner particles are transferred imagewise onto a desired receptor. In the fusing step, the toner is melted and thereby fused to the receptor. An alternative involves fixing the toner to the receptor under high pressure with or without heat. In the cleaning step, residual toner remaining on the photoreceptor is removed. Finally, in the erasing step, the photoreceptor charge is reduced to zero to remove remnants of the latent image.
Two types of toner are in widespread, commercial use. These are liquid toner and dry toner. The term “dry” does not mean that the dry toner is totally free of any liquid constituents, but connotes that the toner particles do not contain any significant amount of solvent, e.g., typically less than 10 weight percent solvent (generally, dry toner is as dry as is reasonably practical in terms of solvent content), and are capable of carrying a triboelectric charge. This distinguishes dry toner particles from liquid toner particles in that liquid toner particles are solvated to some degree and generally do not carry a triboelectric charge while solvated and/or dispersed in a liquid carrier.
A typical dry toner particle generally comprises a visual enhancement additive, e.g., a colored pigment particle, and a polymeric binder. The binder fulfills functions both during and after the electrophotographic process. With respect to processability, the character of the binder impacts charge holding, flow, and fusing characteristics. These characteristics are important to achieve good performance during development, transfer, and fusing. After an image is formed on the receptor, the nature of the binder impacts durability, adhesion to the receptor, gloss, and the like. Polymeric materials suitable in dry toner particles typically have glass transition temperatures over a wide range, e.g., from at least about 50° C. to 65° C. or more, which is higher than that of polymeric binders used in liquid toner particles.
In addition to the visual enhancement additive and the polymeric binder, dry toner particles may optionally include other additives. Charge control additives are often used in dry toner when the other ingredients do not, by themselves, provide the desired charge holding properties. Release agents may be used to help prevent the toner from sticking to fuser rolls when those are used. Other additives include antioxidants, ultraviolet stabilizers, fungicides, bactericides, flow control agents, and the like.
Dry toner particles have been manufactured using a wide range of fabrication techniques. One widespread fabrication technique involves melt mixing the ingredients, comminuting the solid blend that results to form particles, and then classifying the resultant particles to remove fines and larger material of unwanted particle size. External additives may then be blended with the resultant particles. This approach has drawbacks. First, the approach necessitates the use of polymeric binder materials that are fracturable to some degree so that comminution can be carried out. This limits the kinds of polymeric materials that can be used, including materials that are fracture resistant and highly durable. This also limits the kinds of colorants to be used, in that some materials such as metal flakes or the like, may tend to be damaged to too large a degree by the energy encountered during comminution. The amount of energy required by comminution itself is drawback in terms of equipment demands and associated manufacturing expenses. Also, material usage is inefficient in that fines and larger particles are unwanted and must be screened out from the desired product. In short, significant material is wasted. Recycling of unused material is not always practical to reduce such waste inasmuch as the composition of recycled material may tend to shift from what is desired.
Relatively recently, chemically grown toner material has been developed. In such methods, the polymeric binder is manufactured by solution, suspension, or emulsion polymerization techniques under conditions that form monodisperse, polymeric particles that are fairly uniform in size and shape. After the polymer material is formed, it is combined with other desired ingredients. Organosols have been developed for use in liquid toners. See, e.g., U.S. Pat. No. 6,103,781. Some have also been developed for dry toners. See, e.g., U.S. Pat. Nos. 6,136,490 and 5,384,226 and Japanese Published Patent Document No. 05-119529.
Unfortunately, the use of such organosols to make dry toner particles has proved to be substantially more challenging than the use of organosols to make liquid toner compositions. When the organosol is dried to remove the liquid carrier as is necessary to make dry toner particles, the binder particles tend to agglomerate and/or aggregate into one or more large masses. Sometimes, this can be due to the heat required for drying, which causes the particles to melt or soften and thereby coalesce or fuse with other melted or softened particles. Such masses must be pulverized or otherwise comminuted in order to obtain dry toner particles of an appropriate size. The need for such comminution completely defeats a major advantage of using organosols in the first instance which is the formation of monodisperse, polymeric particles of uniform size and shape. Consequently, the full spectrum of benefits that result from using organosols has not been realized for widespread, commercial, dry toner applications.
Particle size and charge characteristics are especially important to form high quality images with good resolution. Dry toner particles must be as uniform in size, charge rate, and charge holding characteristics as is practically possible in order to maximize image forming performance. Accordingly, there is always a demand in this industry for techniques that yield dry toner particles with more uniform particle size, charging rate, and/or charge holding characteristics.